APPROACHING The World Book 1977 with its impressive array of endorsements from educators and librarians, I was skeptical about its place in an average busy American household. Would it not sit neatly and high-mindedly on the shelf along with the gold-tooled editions of the classics that so many of us buy with high hopes and dust off with resignation? The 22 brown and black volumes had no sooner entered my house, been proudly enshelved, than my attitude began to mellow. The younger generation had not even been introduced to it when a friend of mine dropped by. "Oh, The World Book ," she exclaimed, plopping down from the one we used to have at home." She sighed nostalgically and pulled out volume S-SN. "I wonder how much it's changed."

Children were probably culturally deprived three decades ago I thought smugly, beginning a businesslike check for factual updating. CARTER, James Earl Jr. is there already with every detail of his campaign and election; Mao Tse-tung is dead; X-rays have developed. (The World Book has a permanent staff of several hundred engaged on this constant updating.) But as I browsed I bean to understand my friend's affectionate response! One thing, as they say, led to another and I found myself studying some detailed insturctions, fully diagrammed, on how to carve a leg of lamb or a Thanksgiving turkey - there's a lot in The World Book for parents too! And then on to Michelangelo and James Michener, the former gets an appropriate three-and-a-half pages to the latter's two inches. Cross-checking the Mao entry brings me to CHINA, Recent events, and I'm off to another volume to read the latestn the great Peking earthquake.

The World Book is the best-selling encyclopedia of all time acccording to both its publishers and Consumer's Report, full of pictures - many in color - easy to read, and fun to browse. The 22 volumes include a recently added index and research guide. A Year Book is issued in March of each year (costing around $10) to keep owners up to date, although the majority of articles do not date as quickly as those on, say, cancer research or the Republican Party. Greek myths, turkey carving and George Washington will remain current for as long as most families will need them.

An eccyclopedia is a little like a new car - as well as being a major investment, it depreciates the moment it is unpacked. A prospective owner must ask: Is it worth it to me and my children to spend several hundred dollars on a reference book for the home? Can't they go to the library? Of course, salesmen try to persuade us that without an encyclopedia our children will be impoverished, will never begin their life's voyage along the glittering path of knowledge. Guilt is a strong component of parenthood these days and "knowledge" shipped like a golden key to your home and is a very appealing palliative.

But to buy or not to buy is, of course, a question of family interests, income, the availability of librariies, of age and of priorities. Librarians are continually being asked by eager parents of even quite tiny children: "Should we buy them an encyclopedia? Which one? How long will it last? Will it really help them in school?" One very experienced librarian told me that her advice has changed very little over the years. She always advises. "Wait - at least until your oldest child is around nine."

Once you have decided to make the investment, The World Book is hard to beat. Since it was first published in 1917, it has grown steadily until it contains around 10 million words - but not antlike words that make you squint. Accessibility and flexibility are its strengths. A daunting array of experts write the basic articles, but they are edited for clarity and the vocabulary checked against a 44,000 word list prepared by reading experts. Unusual or technical words are always defined.

A good encyclopedia - and without doubt The World Book is one - can become a family friend, a companion for lonely days sick from school, an arbiter of perennial squabbles, the source of the odd and astounding fact that children love to bring out for their friends. A library encyclopedia can be a teacher but not an intimate friend and for this reason, rather than the faultlessly executed theme or painstaking research project, an encyclopedia in the home can become part of the memories of childhood.